1. Field of the Invention
The present invention relates to an interface module in which an electrical-to-optical and optical-to-electrical converter (E/O and O/E converter) and an optical fiber line are used and which is suitable for achieving the long-distance transmission of digital video signals complying with the digital visual interface (DVI) standard.
2. Description of the Related Art
According to reduction in the thickness and increase in the size of video output display units such as plasma display panels (PDPs), liquid crystal displays (LCDs), full-page displays (FPDs), and projectors, such display units have become usable not only as television receivers, but also as, for example, display means for distributing information in advertising and medical fields, public transportation, factories, banks, etc.
Such a video output display unit is often separated from a host device that supplies a video signal (a video signal source, e.g., a personal computer (PC), a display controller, etc.).
When the video output display unit and the host device, which are separated from each other, are connected by a metal conducting cable, a video signal transmitted through the cable may be attenuated and/or reflected. This imposes a restriction on the length of the cable.
A method to solve this problem has been proposed in an interface module using optical fibers, as described in “Optics data sheet M1-201-TR (Ver. 0.9)”.
Referring to FIG. 4A, this interface module includes a transmitting unit Tx connected to a host device (PC) 1, a receiving unit Rx connected to a video output display unit (Display) 2, and four optical fibers F connecting the host device 1 and the video output display unit 2.
Referring to FIG. 4B, the transmitting unit Tx includes an integrated circuit IC1 for processing an image signal and a light emitting device unit L. The transmitting unit Tx connects to a digital visual interface (DVI) connector terminal 3 of the host device 1. The receiving unit Rx includes an integrated circuit IC2 for processing an image signal and a photodetector unit R. The receiving unit Rx connects to a DVI connector terminal 4 of the video output display unit 2. The four optical fibers F, which are arranged in pairs, are attached to connectors so as to be removably connected to the transmitting and receiving units Tx and Rx. The four optical fibers F transmit four signals consisting of RGB signals and a clock signal, which comply with the transition minimized differential signals (TDMS) standard.
In the DVI, which is a proposed standard of an interface for connection with a digital display and electrically is based on the TMDS technology, a display data channel (DDC), which uses an expanded interface that enables the transmission of information from a display to a host device, can be used for identification of a display.
With these four signals, therefore, information cannot be transmitted from the video output display unit 2 to the host device 1, and consequently automatic setting (plug & play function) cannot be implemented. As a result, with this interface module, the identification of the video output display unit 2 cannot be achieved, and the display type and synchronization frequency must be set manually with the host device 1, which is burdensome.
It is conceivable to increase the number of optical fibers F so as to transmit DDC signal, thereby allowing application of plug & play function. In such a case, the transmitting unit Tx must include an additional photodetector unit, and the receiving unit Rx must also include an additional light emitting device unit. This may result in an increase in size and cost.
The optical fibers F have an allowable bend radius. Thus, a space of at least 15 cm is necessary to connect four optical fibers F to the host device 1 in the case where connectors are provided as shown in FIG. 4C. This imposes restrictions with respect to the position at which the interface module can be placed, which is disadvantageous in terms of user-friendliness of the interface module. The larger the number of optical fibers, the larger the space required, and hence the module becomes less user-friendly.
Thus, metal wires M may conceivably be used for DDC as shown in FIG. 5A. In this case, it is effective in terms of usability to use a composite cable H as shown in FIG. 5B, instead of using the metal wires M separated from the optical fibers F. The composite cable H shown in FIG. 5B consists of five metal wires and four optical fibers F. The metal wires M are connected directly to the DDC signal pins of DVI connector terminals 3 of the host device 1 and the DDC signal pins of DVI connector terminals 4 of the video output display unit 2.
With this arrangement, information (DDC signal) from the video output display unit 2 can be directly transmitted to the host device. Thus, it is unnecessary to provide the transmitting unit Tx with an additional photodetector unit, nor is it necessary to provide the receiving unit Rx with an additional light emitting device unit.
In the case of the composite cable, a video signal, which is transmitted through the optical fibers, is not attenuated much, allowing long-distance transmission without problems. On the other hand, in the DDC signal, which is an electrical signal transmitted through the metal wires, attenuation and waveform distortion due to reflection are observed when the transmission of the DDC signal exceeds a certain distance. This causes transmission errors, and the video output display unit cannot be accurately identified.
Since the above-described composite cable deals with an optical signal and an electrical signal, it requires time for termination treatment with a connector for the sake of the different kind of signals. In other words, the optical fibers of the composite cable must be coupled to optical (transmitting/receiving) devices of the connector, whereas the metal wires must be connected to terminal pins. Since different treatments of the termination must be performed, the termination work requires time, the structure of the connector becomes complicated, and the cost for the termination is increased. Also, since the cable size increases, a larger space is necessary for connecting the host device and the display unit.
Furthermore, since the composite cable has a complicated structure and requires a lot of time for termination work, the structure of a connector thereof becomes complicated and the cost is increased in a case where a extension cable is to be connected for extending the composite cable, for example.